A. Technical Field
The present invention relates to a micromechanical sensor comprising a substrate and at least one mass which is situated on the substrate and which moves relative to the substrate for detecting motions of the sensor due to an acceleration force and/or Coriolis force which occur(s), the mass and the substrate and/or two masses which move toward one another being connected by at least one bending spring device, and the bending spring device having a spring bar and a meander, provided thereon, having a circle of curvature whose midpoint is inside the meander.
B. Background of the Invention
Micromechanical sensors are used for detecting accelerations and/or yaw rates along a spatial axis or at least one of three mutually orthogonal spatial axes. The operating principle is basically that a sensor mass is moved relative to a substrate as a response to the corresponding acceleration or yaw rate of the sensor. For this purpose, the sensor mass is movably mounted on the substrate by means of a bending spring device, which is generally composed of one or more bending springs. The design of these bending springs primarily determines the particular directions in which the sensor mass is movable. The spring stiffnesses of the bending springs are different in the individual spatial directions in order to more or less permit different bending directions. This difference in movability may be influenced by varying the cross-sectional surface area of the bending spring, and also by virtue of the spatial course of the bending spring. In particular for a meandering design of the bending spring, relatively high elasticity may be achieved in the plane of the meander. However, shock effects due to impacts to the sensor may cause extreme bending stresses which may result in damage to the bending spring device.
An acceleration sensor is known from U.S. Pat. No. 6,401,536 B1, in which a sensor mass is attached to an anchoring of a substrate by means of a bending spring device. The bending spring device is composed of multiple individual bending springs, which in each case are attached at one end to the anchoring. In addition, at its end facing the sensor mass the bending spring is divided into two branches, each of which is situated on the sensor mass. Each of the branches of the bending spring is curved in a meandering shape, the individual sections extending in parallel to one another. Each turn of the meander is inflected by 180° in a semicircular manner. Depending on the design described, one or more meanders per branch is/are provided. Each of the turns of the meander is such that the midpoint of the particular circle of curvature to which the bending spring conforms is inside the particular meander.
A micromechanical gyroscope is known from DE 698 22 756 T2, in which a sensor mass is likewise attached to an anchoring of a substrate by means of a bending spring device. The bending spring device, the same as in the previously cited document, permits elastic movability of the sensor mass about the anchoring. The bending spring device is composed of three individual bending springs, each of which is curved in a meandering shape. The individual sections of the meander are not oriented parallel to one another. The bending radius of the particular bending spring extends over less than 180° in the corresponding section, so that the arms are spread apart. Once again, the midpoint of the particular bending radius is inside the meander.
One disadvantage of the prior art is that relatively high peak stresses occur in the bending spring devices during extreme deflections of the sensor mass. This may result in damage to the springs, and thus, to the entire sensor. In particular, the springs may break or become torn, thus hindering or completely preventing the movability of the sensor mass.
The object of the present invention, therefore, is to provide a micromechanical sensor which has a movable sensor mass for which on the one hand its movability is controllable, and for which on the other hand even high bending loads may be absorbed at its springs without the expectation of damage.
The object is achieved by a micromechanical sensor having the features of Claim 1.
A micromechanical sensor according to the invention has a substrate and at least one mass which is situated on the substrate and which moves relative to the substrate for detecting linear and/or angular accelerations of the sensor. On the one hand, the mass moves in the sense of a drive motion form, which in the absence of external accelerations is stationary, and on the other hand responds with detection motions when acceleration forces and/or Coriolis forces act on the sensor. The moving sensor mass is attached to the substrate by means of at least one bending spring device. Alternatively, multiple masses which move toward one another may be connected by at least one bending spring and moved relative to one another. Consequently, it is not necessary in each case for the sensor mass to be situated directly on the substrate. In some embodiments of micromechanical sensors according to the invention, the sensor mass may also be attached to a drive mass, for example, and together with the drive mass moved as a primary motion, and moved relative to the drive mass only for indicating an acceleration force and/or Coriolis force. The sensor mass and the drive mass are then connected to one another via the corresponding bending spring device. The bending spring device has a spring bar and a meander provided thereon.
The meander has a radius of curvature having a midpoint inside the meander. A particular elasticity of the bending spring device is achieved as a result of the meandering design of the bending spring device. According to the invention, the bending spring device is designed in such a way that, in addition to the radius of curvature having the inner midpoint, the meander has at least one further radius of curvature having a midpoint outside the meander. The at least one further radius of curvature is situated between the meander and the spring bar. Stresses which occur on the bending spring device are thus reduced. Damage or even breakage of the bending spring device during extreme deflections of the sensor mass are thus avoided. In addition, uniform deflection of the sensor mass is assisted, so that besides the reduction in the risk of damage, the accuracy of the micromechanical sensor in detecting accelerations or rotational motions of the sensor is improved.
In one advantageous embodiment of the invention, the bending spring device has multiple spring bars. When the meander is situated on the spring bar, stresses which occur on the bending spring device may be greatly reduced due to bending which is present. The risk of breakage of or damage to the spring bar is thus reduced.
If the meander is designed in such a way that it merges into the spring bar in a rounded manner, stresses which are caused by bending may be achieved which are more uniform and which do not have unacceptable peaks, even in extreme bending situations. Adjacent components of the meander may in particular be a first and a second spring bar, the sensor mass, the substrate itself, or an anchoring for attachment to the substrate.
Similarly as for the meander merging into the adjacent component in a rounded manner in order to avoid stress peaks, it is advantageous when the spring bar(s) likewise merge(s) in a rounded manner into the adjacent component, in particular the sensor mass or an anchoring for attachment to the substrate. Stress peaks are thus reduced not only in the region of the meander, but also in the remainder of the bending spring device.
In one advantageous embodiment of the invention, another measure for reducing the load on the bending spring device may be achieved by the rounded transition having a non-constant radius of curvature. The meanders as well as the spring bars are thus connected to the adjacent components in a particularly gentle manner with regard to their stresses. The uniformity of the bending and the associated accuracy of the measurement by the sensor are thus improved.
It is particularly advantageous when the rounded transition is elliptical. This also has a positive effect regarding damage and the measuring accuracy of the sensor.
In one particularly advantageous embodiment of the invention, it is provided that the meander and/or the spring bar merge(s) in a branched manner into the sensor mass, the substrate, and/or an anchoring for attachment to the substrate. Stress peaks in the transition points are thus additionally reduced.
When the meander and/or the spring bar has/have a convex curvature, this results in a bending characteristic which reduces stress peaks even in extreme situations, such as mechanical shock events, for example. Damage to the sensor is thus largely avoided.
In one particularly advantageous embodiment of the invention, the bending spring device has multiple meanders which extend in a point symmetrical or axially symmetrical manner with respect to one another. This is advantageous in particular for high bending rates or large expected stresses, since the overall stress may be distributed over the multiple meanders.
A particularly weak inner curvature is achieved when the inflection region of the meander conforms to the inner circle of curvature by greater than 180°. As a result of the large radius of curvature of the inner circle of curvature which is thus made possible, this advantageously causes peak stresses to be distributed over a larger area, thus allowing them to be kept low.
When embodiments of the invention having multiple inner circles of curvature are provided, it is particularly advantageous when the meander conforms overall to the circles of curvature by greater than 180°. In addition, similarly as for a particularly large contact of a single inner circle of curvature, stress peaks are kept low. The individual circles of curvature are connected to linear or also curved sections of the spring.
To obtain a large contact of the circle(s) of curvature, it is advantageous when the circle of curvature of the outer midpoint, or, for multiple outer circles of curvature, the circles of curvature of the outer midpoints, conform(s) overall to the meander by greater than 90°. A concave or convex curvature of the meander is thus achieved.